A: Because this would be a mode without a perfect 5th. (B-F is an augmented 4th. B-G is an augmented 5th. The perfect 5th above B requires an accidental-- that's the only white key for which this is true.) Aloysius seems adamant that the perfect 5th is the interval that establishes the mode in the listener's ear, and so there can be no mode associated with B.

Q: What's the difference between an augmented 4th, a diminished 5th, and a tritone?

Augmented 4th and diminished 5th refer to the same degree in every mode. They are synonyms. "Tritone" refers to this same degree, and is, in an equi-tempered world, a third synonym. Near as I can tell, it has its own name because, in a world of different temperings, there were some keys in which the augmented 4th/diminished 5th was especially hard on the ears. It was sometimes called "the wolf," after the terrifying howl it made. I assume "tritone" or "devil's interval" was reserved for these particularly heinous instances of the augmented 4th (which is not, in equi-temperament, especially hard on the ears).

This answer is extrapolated from an excellent and fascinating article on tuning by Jan Swafford: The Wolf at Our Heels.